A bond angle is a geometrical angle between two bonds originating from the same atom in a covalently bonded molecule, measured in degrees (o).
In other words, it is the angle between three atoms placed sequentially and held by covalent bonds.
The formation of the bond angle between atoms is mainly due to the electrons, both bonding and nonbonding, a concept best explained by the VSEPR theory.
According to VSEPR theory, once a central atom undergoes hybridization and subsequent bond formation, the bonding and the nonbonding electrons arrange themselves to avoid electron-electron repulsions. This is responsible for forming different bond angles and creating various molecular shapes.
The maximum and minimum distance between a set of electrons ranges from 180o to less than 90o. The bond angle of 72o is also seen in polyatomic molecules having considerable strain, for instance- IF7, etc.
Bond angles, therefore, are responsible for organic molecules having three-dimensional shapes; the most common ones are- linear, trigonal planar, tetrahedral, trigonal planar, and octahedral.
Though the nonbonding electrons (or lone pairs) in the central atom are not used in calculating bond angles; however, their presence creates repulsive interactions with the bonding electrons (of the bonds).
So, this brings a further change in the bond angles of the basic shapes to create more varied shapes like bent, T-shaped, seesaw, trigonal pyramidal, etc.
The shapes, in turn, influence various molecular properties like boiling point, reactivity, color, polarity, biological activity, etc.